Breakdown of phosphatidyl inositol (PI) and related phosphoinositides appear to be an early event in IgE mediated degranulation of 2H3 cells since it occurs even when degranulation is prevented by blocking the influx of calcium. The relationship between IgE receptor cross-linking and PI breakdown was examined in the present studies by use of covalently cross-linked IgE polymers. As reported previously, higher oligomers were more active in stimulating histamine release than trimers and trimers more so than dimers. Monomors were inactive. Both PI breakdown and increase in cytosolic calcium levels which were obtained with occupancy of several hundred receptors, were related to the number of receptors occupied. These responses were correlated with release of histamine. However, when histamine release reached a maximum, further breakdown of PI and increases in calcium were observed when additional receptors were aggregated. In the presence of 30% heavy water the rates of both breakdown of PI and release of histamine were increased up to 2.5-fold. Our studies indicate that 1) breakdown of PI is stoichiometrically correlated with the aggregation of receptors, 2) the maximal ability of the receptors to induce release of histamine is not limited by their capacity to induce breakdown of PI and 3) heavy water-well known to enhance secretion-may act at an early stage in the transduction of membrane signal(s) to enhance histamine release.